Wahoo! Not sure when it started, but yesterday I noticed IMAP mail was flowing immediately to my Blackberry Curve (I’m a BIS user on T-mobile). Then today, I see this post over at Blackberry Cool which confirms I’m not the only one!
http://www.blackberrycool.com/2008/09/bis-customers-now-getting-instant-imap-e-mail/
Update: I’m no longer getting emails via IDLE push. I’m back to 15 minute polling. It was fun while it lasted.